Page 273 - Sami Franssila Introduction to Microfabrication

252 Introduction to Microfabrication

6
6
1.0 × 10 A/cm 2
3 AI(2%Cu) 80 0.36 MA/cm 2
10 2 AI(0.5%Cu) 0.55 MA/cm 2
Pure Al
4 60
2 0.74 MA/cm 2
t (h) 8 ∆ R (Ω) 40
3
10 0 20 0.3 MA/cm 2

6
2 0
W/Ti/AI(2%Cu)/Ti Line-W stud
10 −1
14 17 20 23 26 0 200 400 600 800 1000 1200
−1
−4
1/T (10 K ) Time (h)
(a) (b)
Figure 24.17 (a) mean time to failure of 2.5 µm wide Al, Al (0.5 wt% Cu) and Al(2 wt% Cu) lines at different
2
temperatures with 1 MA/cm current density. Reproduced from Hu, C.-K. et al. (1993), by permission of AIP. (b)
incubation time before resistance increase sets in at 255 C. From Hu, C.-K. (1995a), by permission of Elsevier
◦
◦
change when 300 C PECVD is done. This elongation Include design features such as spacing rules
corresponds to stresses over 1 GPa (the order of mag- and dogbones.
nitude can be estimated by Equation 4.1). Aluminium 3. Create a fabrication process for the platinum silicide
lines expand during PECVD, and they are fixed at Schottky diode shown below. Platinum silicide is
their elongated state because of mechanical stiffness of formed by metal/silicon reaction, not by etching.
deposited oxide/nitride layers. This high tensile stress From Chen, C.K. et al: Ultraviolet, visible and
can be relaxed by cracks, and once a crack is formed, it infrared response of PtSi Schottky-barrier detectors
tends to grow. operated in the front-illuminated mode, IEEE TED,
Compressive stresses in aluminium can be relaxed 38 (1991) 1094, fig. 2.
via hillock formation. Hillocks are small protrusions.
Their size can be up to micrometres, which is equal to Al
insulator thickness between two levels of metallization. PtSi SiO 2
If some mechanically stiffer film prevents relaxation in
the vertical direction, then hillocks can grow laterally,
and again, a micrometre is a very typical size for metal p + n n p +
line spacing. In both cases, hillocks can short-circuit p
the two metal lines. Low-temperature processing helps Si
in reducing hillocks (and stress and electromigration).
Alloying aluminium with copper is also helpful in
minimizing hillock formation because it blocks grain 4. How do diffused resistor design rules differ from
boundary diffusion. the thin-film resistor case?
5. Integrated passive chip (Figure 24.13):
24.10 EXERCISES (a) What is the nitride thickness if areal capacitance
2
density is 4 nF/mm , and nitride ε r = 7?
1. How many lithography steps are needed to fabricate (b) Why is the first contact etching by plasma and
the solar cell shown in Figure 1.6? the second by wet etching?
2. Draw the photomasks (e.g., on transparency film) (c) SiCr thin-film resistor resistivity is 2000 µohm-
required to fabricate the RCL chip of Figure 24.13. cm. Design a 5 kohm resistor.